xptemplate has a bazillion features, and yasnippet is primitive in comparison. Just a simple example is that in xptemplate (and probably any respectable snippet plugin in vim) you can type a part of a snippet name, hit TAB, and it will expand the snippet whose partial name you typed (assuming there's no exact match). In yasnippet, you have to type out the full snippet name before it can be expanded. This is pretty annoying, especially for long snippet names.
There are tons of other examples of far more advanced functionality, such as a variety of snippet repetition and embedding features.
This comparative lack of features in yasnippet is pretty surprising, because emacs extensions are usually very feature rich, and everyone seems to love yasnippet. But I think that's because most emacs users don't know what they're missing.
And before someone says it, yes, I know yasnippet gives you the ability to embed eLisp code in it, so I could potentially write all the functionality I need myself. But that's really beside the point. I'm talking about built-in features, not the ability to code up the features myself.